1. Field of the Invention
The present invention relates to a zoom lens system and in particular to a zoom lens system suitable for a digital still camera, and the like using an electronic imaging device.
2. Related Background Art
Recently, a digital still camera has been widespread in which an image of an object formed on an electronic imaging device is output in the form of an electric signal and recorded in the form of a digital image. In the electronic imaging device, with increasing degree of miniaturization and integration, electronic imaging devices with a larger number of pixels have been provided. Accordingly, in a shooting optical system, higher optical performance suitable for an imaging device with a large number of pixels has been desired. At the same time, in view of portability and convenience for a camera, a compact zoom lens has been desired as a shooting optical system. Moreover, in order to correct a camera shake which is a possible cause of terribly deteriorating resolution upon shooting, a zoom lens with a vibration reduction function has been desired as a shooting optical system.
Under these circumstances, a zoom lens system is proposed that is suitable for a digital still camera and the like using an electronic imaging device in Japanese Patent Application Laid-Open No. 2002-277740. The zoom lens system is composed of, in order from an object, a first lens group having negative refractive power, a second lens group having positive refractive power, and a third lens group having positive refractive power. When zooming from a wide-angle end state to a telephoto end state, the first lens group and the second lens group are moved such that a distance between the first lens group and the second lens group decreases, and a distance between the second lens group and the third lens group increases.
Moreover, as a zoom lens system with a vibration reduction function to correct a camera shake upon shooting, a zoom lens system has been proposed in Japanese Patent Application Laid-Open No. 10-161024, in which a portion of a lens group composing the zoom lens system is moved in a direction perpendicular to the optical axis to correct variation in an imaging position caused by a camera shake.
As described above, with increasing degree of miniaturization of an electronic imaging device in recent years, optical performance desired to a shooting optical system becomes higher. For example, chromatic aberration generated in a shooting optical system becomes a blurred color spread over few pixels to ten-odd pixels in a digital image generated after imaging an object on the imaging device resulting in deterioration of resolution.
However, in a so-called negative-leading type zoom lens system in which a lens group having negative refractive power is disposed as the most object side lens group, when the zoom lens system is made to be a high zoom ratio with securing compactness, generation of chromatic aberration upon zooming becomes conspicuous. In particular, when the zoom lens system is made to be compact with retracting the system to be accommodated in a camera body, since the thickness of each lens group is made thinner as much as possible, the number of lens elements has to be reduced, so that it becomes impossible to sufficiently correct aberrations.
In the zoom lens system disclosed in Japanese Patent Application Laid-Open No. 2002-277740, longitudinal chromatic aberration in the telephoto end state and lateral chromatic aberration in the wide-angle end state are large, so that the zoom lens system cannot sufficiently correspond to miniaturization of the imaging device.
On the other hand, when the imaging device is forcibly made to be miniaturized, a received light amount of each pixel becomes low resulting in lowering the sensitivity. Accordingly, in order to compensate for the lowered sensitivity, exposure time upon shooting has to be long, so that it tends to generate a camera shake and the like.
As described above, in the zoom lens system with a vibration reduction function, a portion of a lens group composing the zoom lens system is moved in a direction perpendicular to the optical axis as a vibration reduction lens group to correct a camera shake upon shooting. Accordingly, when the vibration reduction lens group is moved in a direction perpendicular to the optical axis, in other words, when the vibration reduction lens group is decentered, in order not to generate decentering aberration, chromatic aberration and spherical aberration of the vibration reduction lens group have to be corrected. Accordingly, in a zoom lens system with a vibration reduction function, the number of lens elements becomes large, so that the zoom lens system becomes large.
In the zoom lens system disclosed in Japanese Patent Application Laid-Open No. 10-161024, a front lens group in a second lens group is made to be a vibration reduction lens group. However, in spite of a small zoom ratio, deterioration in optical performance is large upon decentering the vibration reduction lens group for correcting a camera shake.